Vulcanized (per)fluoroelastomers have been used in a variety of applications, in particular for manufacturing sealing articles such as oil seals, gaskets, shaft seals and O-rings, because of several desirable properties such as heat resistance, chemical resistance, weatherability, etc.
A frequently used method for manufacturing (per)fluoroelastomers involves aqueous emulsion polymerization of one or more fluorinated monomers generally in the presence of fluorinated surfactants. It is well known in the art that the use of inorganic initiators, e.g. persulfates, generally lead to the formation of polar end groups in the polymer chain, because of the initiating mechanism; among said groups, mention can be made of SO3− groups, carboxylic groups in whichever form, hydroxyl groups and the like. These groups, even if present in low concentration, are generally considered as main responsible for thermal or oxidative lack of stability and/or for interfering with curing processes.
Organic initiators are thus generally initiators of choice when polymers free from polar end-groups and thus more thermally stable and having a lean curing behaviour are to be manufactured by emulsion polymerization.
Nevertheless, when producing fluoroelastomers, more particularly vinylidene fluoride (VDF) fluoroelastomers, by emulsion polymerization in the presence of said organic initiators, the formation of radicals is triggered by thermal decomposition of these latter. Nevertheless, often, these organic compounds have no appreciable decomposition kinetic at temperatures below 100° C.; thus, polymerization conditions required for achieving suitable polymerization rate might be as high as 120° C. or more. As a consequence of these ‘high T’ conditions, polymer chain is submitted to various side-reactions, including rearrangements, back-bitings, transfer to polymer and the like, finally resulting in chain defects, which might compromise mechanical and tensile properties of the fluoroelastomer matrix and processing behaviour of the fluoroelastomer and of its compounds
In particular, it is known that when polymerizing VDF at high temperature, polymer backbone is generally interrupted by short chain branches terminated by end groups of formula —CF2H and/or —CF2CH3, which typically originate from intra-chain transfer (back-biting) as shown, e.g. in scheme here below:

Further, in addition, transfer to polymer phenomena involving attack on —CH2— moieties of VDF recurring units might additionally generate, in particular at high temperature, long chain branches and additional —CF2H end groups, as shown in scheme herein below:

On the other side, polymerization techniques based on the use as radical initiating system of a redox system, comprising at least an oxidant and a reducing agent enabling easier decomposition of said oxidant are known.
Thus, EP 0461521 A (DAIKIN IND LTD) 18 Dec. 1991 discloses a process for manufacturing VDF polymers by certain radical polymerization processes. Among suitable techniques, mention is notably made of suspension polymerization techniques, including the use of organic radical initiators; or, as an alternative, of emulsion polymerization techniques using any of ammonium persulfate, potassium persulfate, hydrogen peroxide, or a mixture of any of these inorganic peroxides with a reducing agent such as ferrous sulfate and a stabilizer such as I-ascorbic acid.
Also, EP 1591461 A (ARKEMA INC) 2 Nov. 2005 discloses the emulsion polymerization of VDF in the presence of certain hydrogenated surfactants in combination with a radical initiator. Said radical initiator can be a redox system made of an oxidizing agent, a reducing agent and a promoter acting as an electron transfer medium; the oxidizing agent can be notably a hydroperoxide, such as ter-butyl hydroperoxide and cumene hydroperoxide; the reducing agent can be ascorbic acid; the promoter can be a transition metal salt, such as notably ferrous sulphate.
Furthermore, use of these redox systems is extremely delicate and mere addition of oxidizing/reducing agent might lead either to no polymerization at all or, under certain cases, to uncontrolled kinetics and/or substantial fouling phenomena, so that the expected aim, i.e. effective production of a fluoroelastomer having substantially no polar group and regular defect-free structure cannot be achieved.
There is thus still a current shortfall in the art for method for manufacturing fluoroelastomers, in particular VDF-based fluoroelastomers, possessing substantially no polar end group, thus being endowed with improved thermal and oxidative stability, and more ordered structure, with limited defects.